<!DOCTYPE html>
<html >
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title> css选择器 </title>
    <style>
        h2{
            color: green;
        }
        .box{
            color: red;
        }
        #block{
         color: blue;
        }
    </style>
</head>
<body>
    <h2 class="box">你好</h2>
    <h2 id="block" class="box">我好</h2>
    <h2>大家好</h2>
</body>
</html>